Customize, Mark article as read

By default, this option is activated. You can change it from the "customize" interface. While you read a channel or feed you will notice that as you scroll down, the articles at the top, are greyed out. When an article is greyed out, it means the systems thinks you have read the article, and it won't show that article ever again. This is helpfull if you have tons of feeds and you don't read everything each time you go to that page. For example, if you have 400 unread articles, and you start reading the first 10, than you come back later, the 10 articles you just read, won't appear again, so you won't have to scroll down and see where you've left your reading.
The downside of this is that the articles you've read, won't appear again. So, make sure to bookmark your article if it's something you wish to return later at.